Optical fiber communications networks are rapidly proliferating in the late 1980's. These networks have a number of distinct advantages over wire-based communications networks, not the least of which is much greater effective bandwidth. In such networks a coherent light beam is intensity modulated by a wide bandwidth spectrum carrying multiple channels. The modulation spectrum may extend, for example, from 40 MHz to 400 MHz, with channel center frequencies spaced every 20 MHz throughout this spectrum. An FM modulated carrier nominally at the center frequency of a channel carries a wideband signal, such as analog video information, over an 18 MHz useful bandwidth. Thus, the coherent light energy of the network is amplitude or intensity modulation carrying plural FM modulated subcarriers. Plural AM subcarriers may also be carried on optical fiber communications networks of the type contemplated herein.
The light energy transferred through a single mode optical fiber network is generated by laser diodes. These iodes are capable of generating about two miliwatts of light energy at an oscillating wavelength of e.g. 1300 nanometers. Because the diodes are very expensive (e.g. presently priced in a range between $2000 and $10,000 apiece) and because they are susceptible to damage and destruction if overdriven, a variety of automatic gain control protection schemes have been proposed in the art.
The problem of protecting the laser diode against overdrive for surges in a range between 5 nanosecond and 100 microseconds is compounded within a telecommunications network. If, for example, the excitation signal is momentarily removed from the transmitting end of the network, as when a technician switches a plug or connection, the automatic gain systems throughout the network cause the gains of all of the repeater stations to approach their maximum values. When the transmit signal is thereafter reapplied to the network, because of the e.g. 100 microsecond latency of each AGC circuit, a significant burst of energy is put out by each laser diode during the interval between first reapplication of the signal and response by the AGC circuit, with damaging and sometimes destructive consequences.
Excessive excitation over time causes excessive heating and stress within the laser diode structure. Since the heating is localized in a very small area, the heat density becomes very high. If the heat energy cannot be dissipated or drawn off fast enough, the diode may be damaged or destroyed. While the laser diode is designed to be capable of withstanding application of significant heat energy for very short time intervals, and is frequently rated at higher power levels for pulsed signal excitation as opposed to continuous excitation, the laser diode cannot handle continuous excessive heat for very long. Damage from excessive heating is manifested in shortened useful lifetime and/or reduced light energy emission. Destruction is characterized by catastrophic failure of the device. While destruction is the usual consequence of extended overload conditions, the more frequently occurring problem, and the one of greatest concern is the gradual degradation of the laser diode resulting from successive non-catastrophic overloads, overloads of moderate duration.
Since laser diodes have been used most frequently within digital networks wherein the information is contained within transitions between discrete levels, fast acting clamping diodes have been typically provided at the laser diode. While clamping diodes do not unduly distort digital information modulation, the clamping diodes introduce unacceptable nonlinearities when the light beam carries analog signal modulation.
Another approach has been to provide laser diode driver circuits which are designed to operate very close to a voltage supply rail, i.e. with very little "headroom". The maximum signal level put out by the driver circuit is thereby clamped to be no greater than the voltage supply rail. These circuits, while entirely satisfactory for digital modulation, are not satisfactory for analog signals which must be kept linear over a much wider dynamic range than digital signals. In order to achieve and maintain linearity within analog systems, amplifiers must have a significant margin for signal handling capability; this margin leads directly to the potential for laser diode overdrive.
Other schemes including optical feedback control loops either have been too slow to provide adequate protection or have not lent themselves to inclusion within optical coupling devices for coupling the laser diode generated light energy to the optical fiber.
Thus, a hitherto unsolved need has arisen for a laser diode protection method within an analog communications network which works effectively over a very short time period in order to prevent excessive excitation impulses from reaching the laser diode without compromise of system linearity.